Orthogonal frequency-division multiplexing (OFDM) has become a key technology in many wireless communication systems. It increases the robustness against frequency-selective fading. By using a cyclic prefix (CP) in an OFDM system, channel distortion can be easily complemented by a simple one-tap equalizer operating in a frequency domain. Frequency domain response of OFDM system is a random variable, in which some subcarriers may be deeply attenuated. The attenuation is typically caused by the frequency selective fading induced by a communication channel. Symbols transmitted in these deeply attenuated subcarriers will have much higher error probability. Accordingly, symbols transmitted on heavily attenuated subcarriers cannot be recovered in a receiver and, thus, they have been transmitted in vain. Thus, limited transmit power resource available at a transmitter has been wasted on these subcarriers.
One solution has been to combine power allocation and error-correcting codes. Assuming the transmitter has knowledge of channel state information, a power control is applied such that symbols allocated to heavily attenuated subcarriers are truncated, and the truncated power could be applied to less-attenuated subcarriers. Symbol erasures affected by the truncating operation could be handled by an error-correcting code.
Known power allocation schemes are based on a prescribed cutoff threshold. The symbols are transmitted only when the channel gain is above the cutoff threshold. Otherwise, no power is allocated for these corresponding symbols. In OFDM systems, the number of subcarriers may be several hundred or thousand, for example. Most of subcarriers in one frequency block may be deeply attenuated at the same time. Accordingly, most subcarriers in the frequency block will be truncated. This induces a very high frame error rate because the impacts of the truncation cannot be solved by an error correction code with a limited block length. On the other hand, if most of subcarriers in an OFDM block attenuate only mildly or do not attenuate practically at all, then this kind of power allocation strategy may not improve the system performance. Therefore, an improved data transmission scheme for OFDM data transmission in fading communication channels is needed.